Hit the high-water mark: Flood frequency estimates to optimize bridge design

Designing bridges effectively — such as to withstand floods — is key to long-lasting and safe structures.

Illinois Center for Transportation and Illinois Department of Transportation seek to better estimate flood frequency in a recently completed joint project, “R27-181: Estimating Peak-flow Quantiles for Selected Annual Exceedance Probabilities in Illinois.”

Neil VanBebber, IDOT’s hydraulics unit chief, led the project along with U.S. Geological Survey’s Thomas Over, Amy Russell, Mackenzie Marti and Padraic O’Shea.

The team aims to help IDOT better estimate flood frequency on Illinois’ rivers and streams as well as to make the data more widely available, allowing agencies to better design and maintain bridges and culverts for current needs.

Key to estimating flood frequency are streamgages, devices that measure the flow of water in a river or stream — a process USGS compares to “taking the pulse” in humans.

Building on data spanning up to 1999 and 2009 from two previous studies, the USGS team evaluated data through September 2017 from 476 streamgages in Illinois to develop flood-frequency estimates.

They used annual maximum discharge data, or largest observed water flow, from each streamgage to estimate the relationships between annual exceedance probability, or the chance different size floods have of occurring each year, and flood magnitude at that location.

They developed a dataset from those relationships as well as a dataset of basin characteristics, such as slope, land use, soil permeability and precipitation, upstream from the streamgages.

Using the two datasets, they developed models to estimate the flood frequency at ungaged bridges and culverts, where most stream crossings occur.

The team packaged the results into a USGS web-based tool, StreamStats, which allows users to easily access flood frequency information in Illinois.

“StreamStats allows a user to predict the peak-flow values of interest at any location in most rivers and streams in the state,” Over said. “Beyond the design of bridges and culverts, which is IDOT’s primary use, another important application of the flood-frequency values provided by StreamStats is floodplain mapping.”

StreamStats also provides information that may be of interest to the general public about streams in their neighborhood.

“StreamStats allows the delineation of watersheds with just a few clicks and provides a set of basin characteristics, such as topographic information like slopes and elevations, certain soil properties, and urbanized fraction, of the delineated watershed,” he added.

The updated tool reflects changes in watersheds and climate, provides more realistic representations of Illinois’ stream network and topography, and includes the effects of urbanization.